When exposed to a radiation such as X-rays, an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) absorbs and stores a portion of the radiation energy. The phosphor then emits a stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In the method, a radiation image storage panel which is a sheet comprising the energy-storing phosphor is used. The method comprises the steps of: exposing the storage panel to a radiation having passed through an object or having radiated from an object, so that a radiation image information of the object is temporarily recorded in the storage panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit a stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus processed is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and an energy-storing phosphor layer provided thereon. However, if the phosphor layer is self-supporting, the support may be omitted. Further, a protective layer is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical damage.
Various kinds of energy-storing phosphor layer are known and used. For example, a phosphor layer comprising a binder and an energy-storing phosphor dispersed therein is used, and a phosphor layer comprising agglomerate of an energy-storing phosphor without binder is also known. The latter can be formed by a gas phase-accumulation method or by a firing method.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. However, it is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and give a reproduced radiation image of high quality (in regard to sharpness and graininess).
It is known that rare earth activated rare earth borate compounds give instant emission off in the ultraviolet or visible wavelength region. For example, L. Zhang et al., “Radiation Effects & Defects in Solids”, vol. 150, pp. 47 to 52 describes that a cerium or praseodymium activated lutetium orthoborate (LuBO3:Ce3+, LuBO3:Pr3+) shows scintillation, namely, gives off instant emission in the ultraviolet or visible wavelength region when excited with UV light or X-rays.
Japanese Patent Provisional Publication Nos. 11-271453, 2001-187884 and 2003-248282 propose utilization of rare earth activated rare earth borates (e.g., GdBO3:Eu, YBO3:Eu) as scintillators or as phosphors for lamps or plasma display panels.
For preparing the rare earth activated rare earth borate, a hydrothermal process is known. For example, Japanese Patent Provisional Publication No. 2001-187884 discloses a process comprising the steps of: preparing an aqueous solution of Y2(OH)3, EU2(OH)3 and H2BO3, adding a basic aqueous solution (e.g., aqueous ammonia) to the former aqueous solution to prepare hydrates; and causing hydrothermal reaction of the hydrates at a predetermined temperature and a predetermined pressure. In the process, it is necessary to make the hydrates gel in the hydration step.
Xiao-Cheng Jiang et al., “Journal of Solid State Chemistry”, vol. 175 (2003), pp. 245 to 251 describes another process which comprises the steps of: dissolving Y2O3, Eu2O3 and H2BO3 in nitric acid, adjusting the pH value of the solution, adding urea to the solution, and subjecting the solution to hydrothermal processing. In the process, an excess of urea often forms undesirable compounds such as Y(OH)CO3 and Eu(OH)CO3. Further, products given by the process are in the form of polydispersive particles.
Yuhua Wang et al., “Chemistry Letters”, vol. 30 (2001), No. 3, pp. 206 to 207 describes a process which comprises the steps of: dissolving Gd2O3, Eu2O3 and B2O3 in nitric acid, evaporating the solution to dryness, and treating the residue hydrothermally.